490 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



movement appears in the order of potency, which is, 

 other things being equal, a time order, or the order of 

 record. The cause of the localization of tissues and 

 structures is much more difficult to understand than 

 the cause of the order of their appearance. The more 

 energetic part of the process naturally requires the 

 greater space for its products. The ectoderm, which 

 becomes the seat of the nervous axis and its muscular 

 adjuncts, occupies the superficial portions of the yolk. 

 Hence, we may regard this expression of the struc- 

 tural record of these functions as more energetic than 

 that of the record-structure of the nutritive functions, 

 which displays itself below the ectoderm. In mero- 

 blastic and amphiblastic embryos, the segmentation 

 which develops the nutritive tissues is evidently more 

 sluggish, for the cells are larger and fewer in number 

 than those of the ectoderm. 



In evolution external stimuli modify the course of 

 emphytogeny above described, and by producing new 

 structural records, cause a new formi of energy, due to 

 composition of the new with the old, and the process 

 of growth then becomes bathmogeny. The external 

 stimuli are molecular or molar, determining physio- 

 bathmism or kinetobathmism. 



The effect of motion or use on the soma may be 

 conveniently termed autokinetogenesis. Moderate use 

 of a muscle is known to increase its size. Irritation 

 of the periosteum is known to cause deposit of bone. 

 Friction and pressure of the epithelium increases its 

 quantity or changes its form. Increased activity of 

 the functions of nervous tissues increases their relative 

 proportions, as in the enlargement of nerves which re- 

 place others which are interrupted by mutilations, etc. 



